Mobile communication systems were developed to provide the subscribers with voice communication services on the move. With the rapid advance of technologies, the mobile communication systems have evolved to support high speed data communication services beyond the early voice-oriented services. In line with this, the mobile data traffic demand is increasing rapidly and therefore there is a need of high data rate.
There are two approaches to improve for increasing data rate: widening frequency band and improving frequency utilization efficiency. In the latter case, however, the frequency utilization efficiencies of the present communication technologies have reached to the logical upper limit already, there is little room for improving the frequency utilization efficiency through technical enhancement. As a consequence, the focus is now on the technology of using a wide frequency band.
Since it is difficult to secure a wide frequency band in the frequency spectrum (<5 Ghz) occupied by the present cellular mobile communication systems, the consideration is taken into the higher frequency band spectrum for broadband service. Since the frequency band available for broadband communication over 1 GHz is limited, discussions are focused on the radio communication in the millimeter wave band over 30 GHz.
However, such a high-frequency band communication has a drawback in that the signal attenuation increases significantly as the propagation distance increases. In detail, as the frequency increases, the propagation pathloss increases and the propagation distance decreases, resulting in reduction of the service coverage. One of the key technologies to mitigate the propagation pathloss and increase the propagation distance is beamforming.
The transmission beamforming is a technology to focus the signals transmitted by a plurality of antennas on a certain direction. An antenna system formed with a plurality of antennas is called array antenna, and the antennas constituting the array antenna are called element antennas or antenna elements. With the use of the transmission beamforming technique, it is possible to increase the signal propagation distance and reduce interference to other users significantly due to the directionality of the signal.
It is also possible to use reception beamforming at the recipient side using a reception array antenna to focus the reception on a certain direction, thereby increasing the signal reception sensitivity and avoiding interference signals in other directions than the intended.
Such a beamforming technique is advantageous for use in the high frequency band communication system. Since the wavelength is shortened as the frequency increases, it is possible to implement an array antenna with large amount of antennas within the same area by arranging the antennas at half wavelength interval. This means that the communication system operating on the high frequency band can achieve relatively high beamforming gain (antenna gain) as compared to the communication system operating on the low frequency band.